Point-contact-electrode driven microwave electroacoustic transducer

ABSTRACT

Improvement of the interconnection between an electroacoustic transducer and a microwave circuit to reduce the insertion losses. A flexible metallic strip connected to the microwave circuit is maintained in contact at a selected chosen point with the transducer under the pressure exerted by a mechanical member adhesively bonded to the said transducer.

United States Patent [191 Chiron et al.

1 51 Apr. 3, 1973 [54] POINT-CONTACT -ELECTRODE DRIVEN MICROWAVE ELECTROACOUSTIC TRANSDUCER [75] Inventors: fiernard ijhiron; Jean Puyhaube rt,

both of Paris, France [73] Assignee: Societe Lignes Telegraphiques et Telephoniques, Paris, France 22 Filed: June 14, 1971 21 Appl. No.: 152,670

52 US. Cl ..333/30 R, 29/2535, 29/493 51 Int. Cl ..H03h 9/30, H04r 17/00 [58] Field of Search ..3l0/8, 9.3, 9.4; 324/56; 29125.35; 333/30 50 References Cited UNITED STATES PATENTS 2,784,481 3/1957 Kitterman .333/72 3,401,283 9/1968 Curran ..3l0/9.7 3,465,177 9/1969 Winslow et al ..3l0/8 fiowave Circu' FOREIGN PATENTS OR APPLICATIONS 1,553,043 12/1968 France ..333/72 OTHER PUBLICATIONS May: Thickness Shear Mode BaTiO Ceramic Transducers for Ultrasonic Lines, 1959, IRE Nat.Conv.Rec. part 6, pp. 24l-25l.

Primary Examiner-Herman Karl Saalbach AtromeyKemon, Palmer & Estabrook [57] ABSTRACT Improvement of the interconnection between an electroacoustic transducer and a microwave circuit to reduce the insertion losses.

A flexible metallic strip connected to the microwave circuit is maintained in contact at a selected chosen point with the transducer under the pressure exerted by a mechanical member adhesively bonded to the said transducer.

3 Claims, 4 DrawingFigures acoustic medium.

bond

PATENTEUAPM I973 725, 25

SHEET 2 OF 2 Attenuation in dB 80 direct leak 70 1 Echo Delay Signal 30 l l l Fig;4 F in GHZ POINT-CONTACT-ELECTRODE DRIVEN MICROWAVE ELECTROACOUSTIC TRANSDUCER BACKGROUND OF THE INVENTION The electroacoustic delay lines commonly employed at ultra-high frequency consist essentially of one or two transducers associated with a crystalline medium, across which the acoustic waves travel at a speed much lower than in air. In transmission type delay lines a employed are cadmium sulphide or zinc oxide (hexagonal crystals) layers as quite monocrystalline as possible. The yield of the conversion depends upon the structure of the layers, and many investigations have been made for the purpose of studying'and improving the yield of the conversion. The materials most commonly employed for the transmission (at reduced speed) of ultra high-frequency sound waves are crystalline materials of the ruby or corundum type. The duration of the delay depends essentially upon the length of the path of the acoustic waves through the material.

The technique most currently employed in fabricating a transducer is a technique of slowly producing layers in vacuo or under a reduced pressure, either by evaporation or by cathode sputtering. The transducer takes the form of a thin film whose thickness, is chosen according to the frequency of the electric energy to be converted. It is of the order of a micron in the frequency ranges most currently employed in the case of the two aforesaid materials (several GHz). On this mechanically very fragile layer an electric contact is to be designed which will couple the electromagnetic energy transmitted by or to an electric circuit to the transducer. Coupling is often a thin gold film vapor coated over the whole of the useful surface of the transducer layer as described for instance in French Pat. No. 1,553,043. Coupling is achieved with the whole of the face of the transducer. As is well known, the electroacoustic yield of the transducer depends very substantially upon a number of physico-chernical characteristics of the layer (stoichiometry, orientation of the crystal axes in relation to the plane of the layer, etc). It is practically impossible to obtain complete homogeneity of the layer over the whole of its surface (a few square cm). This necessarily results in a reduction of the total yield in relation to the optimum value of the yield at the best point of the layer.

BRIEF DISCLOSURE OF THE INVENTION The object of the invention is to improve the overall yield of a delay line, that is to reduce the insertion loss by improving the coupling between the electronic circuit and the transducer.

The present invention consists in employing a flexible point contact whose position on the surface of the transducer is experimentally determined and by pressing the said flexible contact onto the transducer layer by means of a mechanical member which does not interfere with the propagation. The position of said contact is clamped when the optimum point has been found by adhesive bonding. In a preferred variant of the invention, the definitive attachment of the flexible contact is effected under pressure. In accordance with another feature of the invention, the pressing member is a capillary tube consisting of a low loss dielectric material.

The use of a point contact to couple between the electromagnetic energy into the transducer affords the above-mentioned advantage, i.e. the possibility of selecting the point of maximum sensitivity on the layer. The yield of the conversion is increased as compared with the yield of conventional structures.

When the electromagnetic energy is pulsed, the invention also provides the advantage of making it possible to choose the order of the echo which constitutes the output signal. Thereby adjustment of the delay time may be obtained. It is well known that a pulsed signal travelling through a solid medium gives rise to a number of echoes by successive reflections, because an appreciable fraction of the energy which travels through the solid medium at low speed is reflected at the output interface and is propagated a second time, in the opposite direction, through the said medium. In the case of a transmission type delay line, the main or first echo is that which corresponds to the energy which has passed once through the crystalline medium, the second echo corresponds to that fraction of the output energy which has travelled three times through the length of the medium,,and so on. It is obvious that the relative levels of the successive echoes become con-. stantly lower. However, in some cases, the level of-the second echo may be sufficiently close to that of the first to be objectionable when the incoming signals cover a considerable dynamic range. The same phonomenon occurs in the case of reflection type lines. It is obvious that the amplitudes of the successive echoes are added together in the output signal when the coupling affects the whole of the transducer layer. Means have been proposed for eliminatingthe echoes of higher order, generally consisting in slightly inclining the terminal faces of the volume of the medium in which the acoustic waves travel. It is well known, as explained notably in French Pat. No. 1,456,515, that this method may make it possibleto shift the secondary echoesout of the area of the transducer. The aforesaid Patent utilizes this physical property with a view to eliminating the disadvantage resulting from the variation of the delay introduced by the line as a function of the frequency of the acoustic wave which has been propagated. The same solutions have been employed for eliminating by displacement the secondary echoes. However, the inclination of the terminal faces results in an additional insertion loss. In order to deflect the secondary echoes out of the area occupied by the transducer, this inclination may reach a relatively high value in the case of short lines. The use of a point contact for coupling to the transducer element in accordance with the invention limits to a very small area that zone of the transducer which is coupled to the electromagnetic input or output circuits. Any small inclination of the terminal faces of themedium serving for the propagation makes it possible to limit the coupling of the out- DETAILED DESCRIPTION OF THE INVENTION The invention will be more'readily understood with reference to the following description and to the figures accompanying it, which are given as a non-limiting illustrative example of a delay line made of cadmium sulphide transducers, and using corundum as the propagation medium for the acoustic waves. This choice has no limiting character in regard to the invention, wherein FIG. 1 illustrates a coupling device according to the invention between a strip circuit and the delay line as an experimental set-up.

FIG. 2 illustrates an electric characteristic of a line as a function of the inclination of the end faces of the medium.

FIG. 3 is a diagram illustrating the reflection of the acoustic signals.

FIG. 4 illustrates the characteristics of a line according to the present invention.

There is illustrated at l in FIG. 1 the insulating substrate of a microstripmicrowave line whose conductor 2'is shown in section. Welded to the end of the conductor 2 is a flexible whisker 3 consisting, for example, of a gold strip whose end is bent over so as to increase its resilience. In a particular embodiment, the whisker 3 consists of a strip having a width of 0.5 mm, a thickness of 0.05 mm and a length of 5 mm. The bent-over terminal portion of the whisker is applied to the layer 4 constituting the input or output transducer of the delay line. The layer 4 consists of a thin cadmium sulphide layer. The propagation medium 5 consists of corundum. The whisker 3 which is welded to the microwave circuit 1-2 is applied against the transducer layer 4 by the pressure transmitted by member 6. The welding between 2 and 3 is only achieved in the experimental set-up. The delay line, once mounted is terminated by two plugs (or one in the case of reflection type lines) to allow interconnection with input and output microwavecircuits. The plugs are not shown in the figure. In order not to interfere with the microwave field, it is essential for the material of member 6 to be an insulator. In order not to damage the whisker, it is necessary for the end of the mechanical member 6 to be rounded. In a particular embodiment glass is used which has the further advantage that its end may be made as small as desirable in order to reduce the contact area with transducer 4. The optimum point of contact is found in the following way the member 6 is mounted on a micro-manipulator under an adjustable pressure. The operator follows by means ofa binocular magnifier the displacement (over the whose surface of the whisker) which he imparts to the member 6 by actuation of the micro-manipulator. The delay line 4-5 and the microwave line l-2 are interconnected in the manner already known to the person skilled in the art,

p with a test unit comprising an oscilloscope which displays the echo. The operator may thus choose the point of maximum sensitivity by systematically scanning the surface of the transducer 4 by moving whisker 4 on the transducer layer. Once the selectedpoint has been detected, the whisker 3 is locked in the desired position in relation to the layer 4 by depositing a drop of adhesive, which is, for example, slid along the member 6 so that it runs over the whisker 3.

This drop of adhesive is diagrammatically illustrated at 7. The nature of this adhesive is in no way critical. Of course, it must harden without heat and must not react with the cadmium sulphide layer 4. Araldite-based epoxy bonding agents currently marketed have given good results. However, the Applicants have found that a better temperature resistance is obtained by adding alumina powder to the current commercial products, by mixing approximately equal weights of the two materials. When the adhesive has hardened, the member 6 is broken in the neighborhood of its end, as illustrated at 6' in FIG. 1. In a particular embodiment, the member 6 consists of a glass capillary tube having a diameter of about 0.3 mm, whose end has previously been rounded by heating. In the foregoing, it has been assumed that the choice of the point of contact of the whisker on the transducer 4 is determined solely by the sensitivity of the layer at this point.

As has been. stated in the foregoing, the main advantage of the point coupling according to the present invention as compared with whole area coupling resides in the utilization of the optimum characteristics of the transducer, which results in a reduction of the insertion loss,-as is shown by the following results. A reflection type delay line operating is fed by'a hom, the opening of which is a circle having a diameter of 1.5

mm, to' achieve coupling withthe whole transducer area. The insertion loss of such a device is 40 dB. When the-same delay line is fed by a coupling of the type illustrated in FIG. 1, at a selected point within the previously excited transducer area, the insertion loss measured at the same frequency is 35 dB. There was thus obtained a reduction of 5 dB of the insertion loss with respect to prior art coupling.

1 Other factors may determine the choice of the coupling point on the surface of the transducer as for instance multiple echoes in the case of pulsed signals.

As is well known, the level of theundesired echoes may attenuated by inclination of the terminal faces of the medium 5 in which the acoustic energy is propagated, as illustrated in FIG. 3. There are denoted by 0, and 0, respectively the angles of the normals to the inlet and outlet faces in relation to a horizontal. It is to be understood that theseangles have been greatly increased for the sake of the clarity of the illustration. The transducer is shown at 5. In the case of distributed coupling over the entire area of the transducer, it is necessary for collecting solely the first echo E corresponding to a signalv E to the exclusion of the second signal E for 0, and 0, to have a certain value, taking into account the length of the line. Curve 2 shows the values of the incident'angle 0, of the waves at different lengths of medium 5 to obtain a relative attenuation of 18 dB between the second and first echoes. The ordinate axis is in minute of angle (d/60). As appearsa very small variation in 0 has a large influence on the attenuation which would require a machining of the face of so high precision throughout the terminal face area as render it impractical.

In accordance with the present invention, the excitation of medium 5 is effected at a selected point of the surface 4 of the transducer (as well as the capture of the reflected energy) illustrated at E in the diagram of FIG. 3. It will be observed that the point of impact of the energy relative to the first echo E is located in the immediate neighborhood of E E shows the impact of the third echo. The distance E E is a function of the angles and 0, of the two faces. Experience shows that the insertion losses, i.e. the attenuation between the first echo and the input signal, increases rapidly as a function of 0, and 0,. The use of the point contact makes it possible to remove the point E out of the zone affected by the contact between the whisker 3 and the layer 4, by using very small angles 0,, and 0,, as illustrated in the curve of FIG. 2. The scale of the ordinates is the minute of angle. FIG. 2 illustrates the relative inclinations of the terminal faces in a trapezoidal structure as a function of the delay, ie of the length of the medium 5 through which the acoustic wave travels, for an amplitude ratio of 18 dB between the first and second echoes in a transmission type line. It will be observed that these inclinations are very small and consequently cause smaller insertion losses than conventional structures for the same performance.

Experience has shown that the use of a point contact as the coupling with the transducer has another beneficial effect of the insertion loss. Indeed, in previous art devices with coupling on the whole transducer-area, each echo of the acoustic wave impinging on the outlet face of the propagation medium is converted into an electrical wave by the transducer and the electrical energy is transmitted to the output circuit through the coupling. In the case of punctual coupling only that part of the converted energy which appears at the coupling point will be taken away while the energy which develops at any other location within the transducer will not be transmitted to the output circuit and therefor is reconvertedinto acoustic energy to be sent back in the medium. This effect is particularly noticeable in wide band devices.

FIG. 4 illustrates the characteristics of a transmission pending upon the length of the ruby bar, the delay obtained may vary between 0.3 and 10 us. The curves of FIG. 4 illustrate, as a function of the frequency, in the l to 2 GHz band, the relative levels of the delayed signal (lower curve), of the first echo, i.e. the signal which has been reflected at the output end and sent back to the input (central curve), and of the direct leak (upper curve), i.e. the level of the signal directly transmitted (without conversion into ultrasonic energy) between the input and the output of the delay line for a line having a delay of 1 us.

What we claim is: l. A microwave or U.l-I.F. circuit comprising: an electromagnetic transmission line; an acoustic delay line including an acoustic medium having at least one thin film transducer on a face of said medium; means interconnecting said acoustic delay line with said transmission line including a resilient conductive strip having one end'curved; a dielectric rigid cylindrical member positioned to urge said curved end of said conductive strip into engagement with said thin film transducer;

means bonding said dielectric member and said strip to said transducer; and I means for connecting the opposite end of said strip to said electromagnetic transmission line. 2. A microwave or U.I-I.F. circuit as defined by claim 1 in which said rigid member is a glass capillary tube having a rounded end.

3. A microwave or U.I-I.F. circuit as defined by claim 1 in which said rounded end has a surface area of some tens of square micrometers.

i l t l 

1. A microwave or U.H.F. circuit comprising: an electromagnetic transmission line; an acoustic delay line including an acoustic medium having at least one thin film transducer on a face of said medium; means interconnecting said acoustic delay line with said transmission line including a resilient conductive strip having one end curved; a dielectric rigid cylindrical member positioned to urge said curved end of said conductive strip into engagement with said thin film transducer; means bonding said dielectric member and said strip to said transducer; and means for connecting the opposite end of said strip to said electromagnetic transmission line.
 2. A microwave or U.H.F. circuit as defined by claim 1 in which said rigid member is a glass capillary tube having a rounded end.
 3. A microwave or U.H.F. circuit as defined by claim 1 in which said rounded end has a surface area of some tens of square micrometers. 